Mitsubishi Electric ADVANCE Vol.84

VOL. 84/SEP. 1998

Mobile Communications Edition

MITSUBISHI ELECTRIC A Quarterly Survey of New Products, Systems, and Technology ●Vol. 84/Sep. 1998 Mitsubishi Electric ADVANCE

Our cover photographs show the GSM-type MT-30 digital mobile telephone. This lightweight (170g) and compact (135 x 48 x 26mm) unit provides up to two hours of continuous conversation and 120 hours in standby mode using standard batteries, performance that Mobile Communications Edition puts it in the top class worldwide. CONTENTS Editor-in-Chief Shin Suzuki

TECHNICAL REPORTS Editorial Advisors Jozo Nagata OVERVIEW Toshimasa Uji The Present and Future Trends of Mobile Communications ...... 1 Keisuke Ueki by Michio Nakanishi Masakazu Okuyama Yasuo Kobayashi The Development of Digital Mova D203 Hyper ...... 2 Masao Hataya by Tetsuaki Oga and Shin?ichi Fukui Toshiharu Nozawa Seiya Inoue GSM and PCS1900 Digital Cellular Phones...... 4 Takashi Nagamine by Takayuki Nonami Hiroaki Kawachi Development of Model MT151 Mobile AccessTM Phone ...... 7 Akihiko Naito by Kiyoshi Takahashi and Akira Matsumoto Kouji Ishikawa Shingo Maeda A W-CDMA Modem for Mobile Toshikazu Saita Next-Generation Communication Systems...... 9 Akira Inokuma by Takahisa Aoyagi and Hideshi Murai Vol. 84 Feature Articles Editor Viterbi Equalization Technologies for Mobile Broadband Systems ...... 12 Tadashi Fujino by Hiroshi Kubo and Keishi Murakami Editorial Inquiries An Improved Differential Detection Scheme Masakazu Okuyama Based on Maximum Likelihood Sequence Estimation ...... 14 Corporate Total Productivity Management by Toshiharu Kojima and Makoto Miyake & Environmental Programs Low Bit-Rate Speed Coding Technology ...... 17 Mitsubishi Electric Corporation by HirohisaTasaki and Shin?ya Takahashi 2-2-3 Marunouchi Chiyoda-ku, Tokyo 100-8310, Japan Fax 03-3218-2465 SPECIAL REPORT Recent Topics in the ITU-R Relating to Product Inquiries Mobile Radio Communications ...... 20 by Masayoshi Murotani Yasuhiko Kase Global Strategic Planning Dept. Corporate Marketing Group MITSUBISHI ELECTRIC OVERSEAS NETWORK Mitsubishi Electric Corporation 2-2-3 Marunouchi Chiyoda-ku, Tokyo 100-8310, Japan Fax 03-3218-3455

Orders: Four issues: ¥6,000 (postage and tax not included) Ohm-sha Co., Ltd. 1 Kanda Nishiki-cho 3-chome Chiyoda-ku, Tokyo 101-0054, Japan

Mitsubishi Electric Advance is published quarterly (in March, June, September, and December) by Mitsubishi Electric Corporation. Copyright © 1998 by Mitsubishi Electric Corporation; all rights reserved. Printed in Japan. TECHNICAL REPORTS Overview The Present and Future Trends of Mobile Communications

by Michio Nakanishi*

The systems responsible for mobile communications are increasingly adopting digital technologies, and expectations are rising for the move from voice communications to multimedia communications. Public systems prom- ise to experience enormous demand, a trend supported by increases in living space and the diversification of economic activities. The eager acceptance given to the immediacy and simplicity of mobile communication has resulted in its adoption by users in many different age groups, and it is increasingly important for information communications. By the end of 1997, some 204 million subscribers in more than 100 countries were using mobile telephones. In Japan, mobile telephones including the PHS system had attracted more than 37.6 million subscribers by February, 1998, rapidly approaching the high levels of penetration in the Scandinavian nations. Investigations of and research into mobile telephone systems are underway in many nations, seeking to identify those that will provide more effective means of coping with large numbers of subscribers than the current implementations of cellular phones using GSM, PDC, D-AMPS, etc., and mobile phones using CDMA, etc., already in use in the U.S.A. Among these is the next-generation IMT-2000 (FPLMTS) communications system, which is being prepared for implementation in the year 2000 in Japan, Europe and the U.S.A. Studies of this next-generation system are concentrating on unified national standards that will enable a single handset to function with universal compatibility while at the same time handling multimedia communications. Research is also under way into links between next-generation high-speed radio communications access systems and mobile satellite communications systems. On the other hand, mobile business communications in Japan are about to go over to digital technologies and to progress from voice to multimedia communications. ❑

*Michio Nakanishi is a Director, Communication Systems Group.

September 98 · 1 TECHNICAL REPORTS The Development of Digital Mova D203 Hyper

by Tetsuaki Oga and Shin’ichi Fukui*

Mitsubishi Electric’s easy-to-use Model D203 digital cellular phone is an 800MHz PDC type that features a compact lightweight design, extended battery standby and operating time, and the ability to display text messages (using the Short Message Service) on a four line by ten character LCD panel with electroluminescent lighting for nighttime use.

In 1996, Japan had nearly 21 million digital cell phone users, and the market is continuing to grow. In June 1997, Mitsubishi Electric launched Model D203, developed under the guidance of NTT Mobile Communications Corporation for the company’s 800MHz PDC digital phone service. The cell phone boasts several user-friendly features. Extensive measures to lower power dissi- pation have extended the battery operating time to 90 minutes of continuous speech and 200 hours on standby with a standard battery, or 190 minutes of speech and 420 hours on standby with an L-size battery. Incoming calls can be signaled by an audible ring tone or a silent vibrator, permitting discreet use, while the flip-open handset need not be open to answer calls, simplifying operation and enhancing reliability. Model D203 sup- Fig. 1 Model D203 digital cellular telephone. ports Japan’s short mail service, available since June 1997, displaying messages on a four-line by ten-character screen. A bright electroluminescent backlight illuminates the screen for nighttime use. The product is available in cosmic blue and Table 1 Specifications glossy gray finishes. Fig. 1 shows a photograph of Battery type S L Dimensions the Model D203. 123 x 40 x 26mm 123 x 40 x 38mm (LxWxD) Specifications Weight 120g 155g Table 1 lists the specifications and Fig. 2 shows Battery type Lithium ion a circuit block diagram. The unit measures Nominal operating 3.6V 123mm high, 40mm wide and 26mm thick and voltage

weighs 120g with a standard battery—light- Nominal capacity 600mAh 1,300mAh weight and convenient for handheld operation. The L-size battery, which extends the opera- Charging time 80min 190min tion and standby time, increases the width to Transmit power 0.8W 38mm and the weight to 155g. The flip-open Frequency band 800MHz case was designed to protect the keypad from Talk time (max. power, full rate impacts and accidental operation. 90min 190min speech coding, "off" power saver mode) Transmitter Circuit The I.Q. generator converts serial binary data Standby time 200h 420h

*Tetsuaki Oga and Shin’ichi Fukui are with the Mobile Communication Business Division.

2 · Mitsubishi Electric ADVANCE TECHNICAL REPORTS

RF BLOCK CONTROL BLOCK TRANSMITTER BLOCK

Control signals Transmit/receive IQ GENERATOR BLOCK antenna Transmit power controller allel

I ter RF generator and Quadrature Data er power amplifier Q v

modulator erential ial-par coding Audio Audio con Diff

Nyquist filter processor Ser codec

Transmit/receive Microphone switch Synthesizer

Receive Micro- Keypad and antenna processor LCD display First IF circuit Second IF circuit Memory

Low noise First local IF Second local Mixer Mixer Demodulator amplifier oscillator IF oscillator Power supply

Filter TDMA processing, baseband processing (SAW filter)

RECEIVER BLOCK

Fig. 2 Block diagram.

from a microprocessor in the logical control switches and phase-locked loops. block to parallel data which is then differential coded and passed through a bandwidth-limit- Logical Control Block ing Nyquist filter in preparation for π/4 quadra- This section controls the RF block, processes ture phase shift keying (QPSK) modulation. The signals to and from the base station, performs filter has a root Nyquist roll-off ratio of 0.5 that connection management, decodes keyboard limits the width of the passband. The quadra- inputs, manages the LCD panel, serves as a ture modulator performs direct quadrature speech codec, performs audio passband filter- modulation of the local transmission signal ing and volume control, and manages the con- from the frequency synthesizer, which simpli- nection with a fax-modem adapter. The M30600 fies the circuitry and reduces the component series microprocessor, a monolithic device with count. a 16-bit data path, is used as the processor. Flash A compact two-stage GaAs amplifier has been memory holds the firmware. SRAM and used in the RF block, providing efficient, gen- EEPROM are also used. eral-purpose amplification. Dual FETs are used to implement power control in six, 4dB steps. Display The LCD panel features a space-saving inte- Receiver Circuit grated driver circuit that is fabricated directly A diversity receiver coupled with a double su- on the LCD glass cover using Mitsubishi?s chip- perheterodyne amplifier is used to improve sig- on-glass technology. The electroluminescent nal reception under conditions of poor wave backlight is more than twice as bright as a point- propagation. A surface acoustic wave (SAW) source LED using the same power, and provides resonator in the first IF filter saves space while uniform illumination that makes the display a root Nyquist filter with a 0.5 roll-off in the easy to read in dark locations. second IF filter reduces distortion. Careful engineering has resulted in a compact Frequency Synthesizer lightweight cell phone featuring extended bat- A specially developed compact, power-saving tery operating time and support for text mes- ASIC, contains local oscillators for the trans- sages. The authors would like to express their mitter and receiver circuits in both analog and thanks for the assistance of NTT Mobile Com- digital bands. Produced using a BiCMOS pro- munications Corporation and the many indi- cess, the device includes mixers, amplifiers, viduals involved in the development project.❑

September 1998 · 3 TECHNICAL REPORTS GSM and PCS1900 Digital Cellular Phones

by Takayuki Nonami*

Since the GSM digital cellular phone standard of these versions of the GSM standard. was established in 1991, it has been adopted in Although GSM was originally developed as a about 100 nations and currently serves some common standard for 900MHz-band commu- 60 million subscribers. Mitsubishi Electric has nications in Europe, it also serves as the basis recently developed a lightweight GSM cellular for the 1,800MHz Digital Cellular System (DCS) phone using 3V technology throughout to and the 1,900MHz Personal Communication achieve a standby time in excess of 100 hours. System (PCS) in the United States. Table 2 lists This report introduces Model MT-30 for GSM the specifications of these systems. applications and Model G100 for use under GSM features international roaming support, PCS1900, a GSM-based 1.9GHz-band system a short-message service that transmits text adopted by the United States. messages of up to 160 characters, call transfer and hold functions, conference calling services, The GSM Standard 9.6kbps fax and data transmission, and a sub- The GSM system was developed by the Special scriber identity module—a card containing user Mobile Group of the European Telecommuni- information. cation Standardization Institute (ETSI) based on ISDN network technology and time division GSM Cellular Phones multiplex access (TDMA) mobile communica- Table 3 lists the specifications of Models MT- tion technology. The basic functionality for the 30 and G100. Model MT-30 features extremely system was established in 1991 and Phase 1 compact dimensions of 35 × 48 × 26mm and trials were conducted in Germany in 1992. weight of 170g while delivering state-of-the-art Mobile ISDN functions were implemented in performance: two hours of active operation or Phase 2 trials beginning in 1996. ETSI is plan- 120 hours on standby with the standard battery ning further additions and improvements to the pack. Model G100 was developed concurrently GSM standard that will be tested in a Phase 2+ and has similar specifications. Table 4 lists ac- trial. Table 1 compares the functionality of each cessories available for these two products.

Table 1 GSM Development History Phase 1 Phase 2 Phase 2+

Specifications development 1987~1991 1992~1995 1995~1999

Trial period 1992~1996 1996~1999 1998~2000

Voice transmission modes Full rate Full rate, half rate Full rate, half rate, enhanced full rate

Full rate (11.4kbps max.), Full rate (9.6kbps max.), Full rate (9.6kbps max.), half rate (4.8kbps max.), Data transmission half rate (4.8kbps max.) half rate (4.8kbps max.) packet transmission (76.8kbps max.), high-speed circuit exchange (78.8kbps max.)

Call transfer, call limiting, hold, caller ID, advice of Supplementary services Call transfer, call limiting charge, conference calls Enhanced versions of Phase 2 services (6 parties max.), etc.

Category-specific text Bidirectional text transmission, transmission (basic, voice Phase 2 capabilities plus data packet transfer Short message service cell-specific announcements mail notification), cell-specific and two-byte code transfer announcements

Global roaming, Global and domestic roaming, SIM toolkit, Other Global roaming, 5V SIM card 3~5V SIM card, multiband GSM/DCS support, interoperability DCS1800 specifications with DECT

*Takayuki Nonami is with Mitsubishi Electric France.

4 · Mitsubishi Electric ADVANCE TECHNICAL REPORTS

Table 2 Specifications of GSM and Derivative Systems System GSM DCS1800 PCS1900

Transmit frequency 890~915MHz 1,710~1,785MHz 1,850~1,910MHz

Receive frequency 935~960MHz 1,805~1,880MHz 1,930~1,990MHz

Separation between transmit and 45MHz 95MHz 80MHz receive channels

Access method TDMA-FDD

Multiplex number 8

TDMA frame length 4.615ms

Channel spacing 200kHz

Number of RF carrier channels 124 374 299

Modulation rate 270.833kbps

Modulation type GMSK, BT = 0.3

22.8kbps RPE-LTP, 22.8kbps RPE-LTP, 22.8kbps RPE-LTP, Voice coding algorithm 22.8kbps ACELP, 22.8kbps ACELP 22.8kbps ACELP 11.4kbps VSELP

User data transfer rate 2.4, 4.8 or 9.6kbps

Waveform equalization Up to 16µs

Mobile unit transmission power 250mW avg., 2W peak 125mW avg., 1W peak 125mW avg., 1W peak

Key TDMA-FDD Time division multiplex access, frequency division duplex RPT-LTP Regular pulse excited, long-term prediction ACELP Algebraic code excited linear prediction VSELP Vector sum excited linear prediction

Table 3 Mobile Unit Specifications Table 4 Accessories Model MT-30 G100 Available for US, UK, continental Europe AC adapter and Australia System GSM PCS1900 600mAh S type, 650mAh M type, 135 x 48 x 26mm Battery pack Dimensions 1,150mAh L type with S battery High-speed Weight 170g (S battery) Fits main unit and spare battery desktop charger Supply voltage 3.6V Cigarette lighter For rapid in-car charging Battery capacity 600mAh (S battery) charger

2 hours 3 hours “Hands free” Talk time For hands-free operation while driving (S type, 50% DTX) (S type, 50% DTX) adapter

Standby time 120 hours (S type) Headset Quasi hands-free conversation

22.8kbps RPE-LTP, Data adapter PCMCIA card for fax and data transmission Voice transmission 22.8kbps RPE-LTP 22.8kbps ACELP Data cable RS-232C for SMS, etc. Data transfer rate 2.4, 4.8 or 9.6kbps

Peak transmit power 2W 1W

Transmit frequency 890~915MHz 1,850~1,910MHz

Receive frequency 935~960MHz 1,930~1,960MHz power supply. The RF section consists of a power amplifier for the transmitter, a receiver amplifier and an RF processor for transmit and receive functions. The baseband section consists Externally, the cellular phones consist of a of a microprocessor, DSP and ASICs for flip-open case with a large LCD panel and key- baseband and audio processing. Fig. 2 shows the pad. Table 5 lists the supported functions, which configuration of the system software, which is include a variety of call-transfer and supplemen- based on a realtime multitasking operating sys- tary services in addition to basic voice commu- tem. nication and speed-dialing functions. Fig. 3 shows the configuration of the baseband Fig. 1 shows a block diagram of the MT-30/ processing ASIC developed for this application. G100, which is divided into RF and baseband The device has an analog section with a GMSK sections. Both sections operate on a single 3V modulator and demodulator ADC, and a digital

September 1998 · 5 TECHNICAL REPORTS

Table 5 Mobile Unit Functions 4 lines of 12 characters each, double height Display GMSK Modulator character support, 11 icons Post filter modulator I/Q DAC 4 cursor keys, 4 function keys, 2 volume control Modem I/Q Control panel keys, 12-key touch-tone keypad interface Demodulator Digital filter Prefilter Antenna Fixed 25mm I/Q ADC Control Connectors RF, DC input, headset, SIM, external I/O interface Encryption Voice communication, emergency calls, Wireless link control DAC processing Wireless Communication bidirectional text message transmission, voice- link services mail and other notifications, fax and data control transmission Wireless interface TDMA controller link Call transfer, call restrictions, hold, conference controller Supplementary calls, caller ID, advice of charge, extension services calls and transfers

Fixed and programmable speed dialing memory, Fig. 3 Baseband IC configuration. redial, storage of called party's phone number, Speed dialing storage of non-answering party's phone number, scratch pad memory section with a TDMA controller and a micro- Network selection, display language selection, Other keypad lock, call elapsed time display processor interface. The TDMA controller sup- ports two clock frequencies, 13MHz and 32kHz, and switches to the slower frequency to save power whenever possible. The DSP is a single-chip solution that implements a full-rate regular pulse excited long-term prediction (RPE-LTP) audio codec, an algebraic Power Audio DSP amplifier code exited linear prediction (ACELP) audio IC codec, error-correction coding functions for Automatic Base- RF Duplex power voice and data transmission and received sig- band section filter Control panel IC control nal sync detection. and display Micro- The cellular phone's 3V 2W RF power ampli- processor Low-noise SIM support amplifier fier IC was developed specifically for GSM applications. This hybrid GaAsFET device has a volume of just 0.4cc while achieving better than Fig. 1 Hardware configuration. 50% efficiency.

Mitsubishi Electric plans to develop smaller and lighter GSM cellular phones based on the technologies presented here while tracking other User interface industry trends and continuing to extend battery operating time. ❑

Call controller

"Hands free" control SIM interface

Wireless resources manager

Data link controller

TDMA controller

Realtime operating system

Hardware interface

Fig. 2 Software configuration.

6 · Mitsubishi Electric ADVANCE BACK CONTENTS NEXT TECHNICAL R EPORTS Development of Model MT151 Mobile AccessTM Phone

by Kiyoshi Takahashi and Akira Matsumoto*

The Model MT151 Mobile AccessTM phone is a cellular telephone incorporating software and hardware that provides access to a variety of Internet services while maintaining the lightweight port- ability demanded of handheld mobile phones. This Mobile Access phone implements these capabilities using Cellular Digital Packet Data (CDPD) wireless packet data communications technology and Unwired Planet’s UP.Link application technology.

Mitsubishi Electric’s Mobile Access phone adds CDPD functions and the UP.browser to the standard cellular telephone functions of the United States’ Advanced Mobile Phone Service (AMPS). The stand-alone unit can send and receive email and use various Internet services without con- necting a personal computer. Because the ap- Fig. 1 Mitsubishi Electric’s Mobile Access phone. plications and processing are performed on a web server, the system is highly flexible, allowing corporations to deliver custom user application services over private intranets. The UP.browser the U.S. analog cellular phone system. Because functions are tightly integrated with the cellu- it employs unused channels, the system is eas- lar phone functions, allowing users to dial any ily implemented, requiring little, if any, addi- telephone numbers appearing on a web page tional investment in base-station equipment. without using the keypad. To protect data pri- Service areas have expanded dramatically since vacy, CDPD and UP.Link incorporate encryp- the first field trials were conducted in 1994. tion and authentication functions that operate Some 12% expansion was recorded in the first transparently, without user intervention. six months of 1997, and as of May 1997, ser- The photograph in Fig. 1 shows the Mobile vices were available to half of the U.S. popula- Access phone. The unit features an LCD screen tion. CDPD uses the OSI reference model shown displaying four lines of 12 characters, a 24-key in Fig. 2. Internet connections are supported by keypad with integral touch-tone pad, as well as using the IP protocol in the network layer and antenna, microphone, speaker and battery pack. assigning each mobile telephone an IP address. On the side of the unit is a serial connector for attaching a personal computer and on the bot- tom a connector for the cigarette lighter socket in a car or other accessories. Layer 3 SNDCP MNRP At the heart of the phone is a Mitsubishi 16- Layer 2 MDLP bit microprocessor, type M37702. Programs are RRM stored in flash memory to allow updating. The Layer 1 MAC processor runs a multitasking operating system with realtime control capabilities. Software ex- Key ecution is managed by a realtime monitor. SNDCP Subnetwork Dependent Convergence Protocol MNRP Mobile Network Registration Protocol MDLP Mobile Data Link Protocol CDPD MAC Medium Access Control RRM Radio Resources Management CDPD is a wireless packet data communication system that utilizes open channels in AMPS, Fig. 2 OSI reference model.

*Kiyoshi Takahashi and Akira Matsumoto are with the Communication Systems Development Center.

September 1998 · 7 TECHNICAL R EPORTS

Table 1 Typical UP.Link Based Services HDML Electronic mail, personal information manager, General-purpose news, stock prices, air travel itinerary

Sales information, order status, inventory data, Mobile Access phone Business with UP. Browser client data an HDML browser Internet or Banking services, personal news services, HTML corporate IP net CDPD network Consumer yellow pages, movie schedules, lottery results, browser astrology readings, games

HDML

URL HTML HTTP HDML on a corporate LAN. The HDML information Web server passes through UP.Link Gateway to the

Query Data UP.Browser for display on the Mobile Access UP. Link Gateway phone’s screen. Although the screen displays Database only text information, we believe that the ability to readily access the Internet from a pocket- Fig. 3 Description of a Network using UP.Link. sized terminal will create a new market for handheld information equipment. Table 1 lists typical services that could be delivered by the The CDPD protocol also features a standby mode Internet or a corporate intranet. with intermittent operation that increases mobile phone standby time compared to the stan- The combination of cellular phone and Internet dard AMPS standby mode. access capabilities in Mitsubishi Electric’s Mo- bile Access phone offers to provide mobile trav- UP.Link elers with email connectivity and other data The Mobile Access phone uses the UP.Link com- services without a separate computer. ❑ munications software developed by Unwired Note: “Unwired Planet,” “UP.Link,” “UP.Browser,” Planet. This technology provides Internet access “HDML” and “Handheld Device Markup to cellular phones despite the limitations of the Language” are trademarks of the Unwired small display screen. Fig. 3 shows the configura- Planet Corporation. tion of a network connection implemented using UP.Link technology. UP.Link Gateway serves as an intermediary between the web server and UP.Browser, the Mobile Access phone’s browser application, converting the web’s HTTP protocol to a low-overhead proprietary protocol and keep- ing track of accounting information. The UP.Browser has a restricted feature set designed to operate on a cellular phone with limited memory capacity and modest processor performance. The browser used Handheld Device Markup Language (HDML) in preference to the HTML used in standard web pages and browsers. The Mobile Access phone accesses the Internet via UP.Link Gateway. The server de- livers information coded in HDML to the screen, whether stock prices or weather forecasts from the Internet, or inventory data from a database

8 · Mitsubishi Electric ADVANCE TECHNICAL REPORTS A W-CDMA Modem for Mobile Next- Generation Communication Systems

by Takahisa Aoyagi and Hideshi Murai*

Wideband Code Division Multiple Access (W- (ETSI) considers the field of candidate technolo- CDMA) offers effective frequency utilization gies to be W-CDMA and TD-CDMA. In the and flexibility for variable data transmission United States, CDMA 2000 (an enhanced N- rates, so that it is one of the most promising CDMA known as IS-95) and wideband TDMA candidates among radio access technologies for (enhanced Personal Communication Systems) are International Mobile Telecommunications being considered. 2000. This article describes a prototype W- Mitsubishi Electric has been actively devel- CDMA modem and reports on its field trials. oping W-CDMA technologies, especially coherent detection techniques, variable bit-rate transmission methods, and transmit power con- Trends in IMT-2000 trol technology. The company has also been Cellular telephone systems have been changing working through ARIB on Japanese standard- from first-generation analog types to second-gen- ization activities and has made W-CDMA-re- eration digital types. Third-generation systems be- lated proposals to ETSI. This article introduces ing planned for introduction in the year 2000 an adaptive variable bit-rate transmission sys- should increase capacity over that of the second- tem, and reports on indoor field testing of pro- generation, and will support various data rates, totype W-CDMA equipment. including high data rates for images. The International Telecommunications Union Variable Bit-Rate Transmission (ITU) is responsible for making the standards for We studied an adaptive data rate scheme under third-generation mobile communication systems, which a W-CDMA modem adjusts its data trans- or IMT-2000. In Japan, the FPLMTS research mission rate to suit varying requirements for committee led by the Ministry of Posts and Tele- voice, images or other data stream. Fig. 1 illus- communications and the Association of Radio trates the variable bit-rate system. Industries and Businesses (ARIB) has engaged in IMT-2000-related technical studies since 1994, USING BI-ORTHOGONAL SIGNALS FOR MODULA- and at the end of 1996 decided to adopt W-CDMA TION. Use of this signal format improves trans- in order to propose an IMT-2000 system using mission characteristics, decreasing the required this technology to the ITU. In 1998, the Euro- Eb/No by about 2.2dB compared to BPSK using pean Telecommunications Standards Institute four-bit bi-orthogonal code without FEC under a bit-error rate of 10-5 in static channel. Fig. 2 shows the configuration of the modulator. In this system, the orthogonal code is selected on 128 chip 1,024 chip the basis of the information rate being trans- 38.4kpbs (4 bit) mitted. Transmission data B1, B2 and B3 lead to selection of an orthogonal Walsh signal of 4 bit 64 chip which the sequence length is 8. The sign of the 76.8kbps signal is determined by B0. The demodulator (8 bit) detects the orthogonal code type and sign and 4 bit 4 bit demodulates the transmission data. 32 chip 153.6kbps (16 bit) PROCESSING GAIN MODIFIED FOR TRANSMISSION RATE ≤ 153.6kbps. The chip number used to 4 bit 4 bit 4 bit 4 bit spread the encoded data is set at 32, 64 or 128 to suit the data transmission rate. Fig. 1 Variable rate transmission method. Data rates of 307.2kbps and higher are achieved by multiplexing several of the signals ORTHOGONAL CODE MULTIPLEXING FOR R > shown. 153.6kbps. As shown in Fig. 1, when the trans-

*Takahisa Aoyagi and Hideshi Murai are with the Information Technology R&D Center.

September 1998 · 9 TECHNICAL REPORTS

Bi-orthogonal modulation

Data B3 XOR B2 FHT Serial-to-parallel B1 converter 9.8304 Mcps B0 Code multiplexer Code multiplexer

9.8304Mcps Ch Walsh (sequence length: 32) Common PN code

Pilot

Fig. 2 Block diagram of modulator.

mission rate is 153.6kbps, the bi-orthogonal en- Table 1 Experimental modem specification coded bits are spread by 32-chip PN encoding, Transmission frequencies

so that as shown in Fig. 2, orthogonal Walsh Downlink: 2,150.5MHz signals with a sequence length of 32 are multi- Uplink: 1,990.5MHz plied by a bi-orthogonal signal, permitting multiplex transmission. Since code multiplexing is Power (at antenna) 0.1W performed by orthogonal codes, cross-correla- No. of channels 1 pilot channel, 0~2 data channels tion between multiplexed signals is virtually Multiplexing system Orthogonal code multiplexing

absent, and transmission speed can be raised to Data modulation BPSK/Bi-orthogonal transmission (k = 4) the appropriate level for the number of multi- method

plexed signals. Spreading method BPSK Since the processing gain in the multiplex en- Transmission rate 9.6 x n kbps, for 1 ≤ n ≤ 32 coding process is higher than the fixed defined value, the cross-correlation of the signal with Chip rate 9.8304MHz transmissions from other W-CDMA stations is Processing gain 15~30dB small, which has the effect of distributing and reducing interference effects among signals from nearby transmitters. Since the bi-orthogonal signal is a binary sequence with multiple characteristics and open-loop transmission information bits, fewer multiplex signals may power control characteristics. Table 1 lists speci- be used than in multiplexing of BPSK signals, fications of the test station. Fig. 3 show the delay reducing the envelope variations. profile. The maximum delay is 0.35~0.4µs, so that in this indoor signal propagation environ- Field Testing ment, the RAKE reception observation window We implemented a coherent detection RAKE of 0.4µs is a sufficient condition. receiver using the pilot channel as the phase Fig. 4 shows the open-loop transmission reference and performed field tests in an indoor power control characteristics. The control is environment to determine multipath fading effective in holding the received signal level

10 · Mitsubishi Electric ADVANCE TECHNICAL REPORTS

cus to compact, low-power designs suitable for 80 commercial products. ❑ 70

Correlation value Correlation value 30

0 0 5 10 15 20 25 30 35 Delay time (50ns)

Fig. 3 Delay profile. Data points are at 50ns increments.

Received IF power (dBm) Received IF power (dBm) 0 0 Without control Without control -20 -20 Target value -40 Target value -40 With control -60 With control -60 1.2 0.8 0.4 0 100 200 300 400 Rate1.2 of occurrence0.8 0.4 (percent)0 100 200 300 Time 400 (s) Rate of occurrence (percent) Time (s)

Fig. 4 Performance of open-loop transmission power control. fixed relative to the central level. However, even if the 10MHz W-CDMA signal is used, there is a 10dB fluctuation, so that closed-loop control will also be required.

The technologies and test results presented here use different signal formats and specifications from Japan’s proposals for IMT-2000; however, the transmission power control processing, detection technologies, and variable-bit-rate control technologies provide valuable experience and knowhow for future IMT-2000 systems. Having established basic W-CDMA technologies, Mitsubishi Electric will be shifting its fo-

September 1998 · 11 TECHNICAL REPORTS Viterbi Equalization Technologies for Mobile Broadband Systems

by Hiroshi Kubo and Keishi Murakami*

This article reports on a list-output Viberbi equalizer developed at Mitsubishi Electric for CHANNEL implementing multimedia applications in a Transmitted ... mobile environment. The authors propose an signal T T T algorithm for selecting and combining metric criteria based on estimated channel character- c0 c1 c2 ... cL istics and developed an algorithm for high-speed Noise compare-select operation. + +

Multimedia data transmissions in a mobile Received signal Decisions communication environment require high qual- Viterbi equalizer ity at a high data rate. However, increasing the data transmission rate leads to serious performance degradation due to increased vulnerabil- Fig. 1 Communication system model. ity to fading effects caused by reflections and waveform distortion effects caused by variations in signal propagation speed along the transmis- shorter than the transmission line memory sion path. Specifically, at data rates above length L, refering to the survivor path for its 10Mbps, delay effects can span from ten to sev- branch metric generation. For each state, several tens of bits. Viterbi equalizers and other eral survivor paths are retained. Decision feed- adaptive equalizers can be effective under these back sequence estimation (DFSE)[4] uses the first conditions; however, correcting dispersion ef- generalization. Complexity increases when the fects at higher data rates requires additional number of survivor paths S is large because of hardware complexity with reduced processing the sorting required for a list-output Viterbi delay at the same time. equalizer.

Communication System Model Simulation Model Fig. 1 shows the communication system model. For the fading simulation model, we assumed The channel is assumed to be a transversal fil- that channel coefficients have equal averages, ter. The channel tap coefficient for memory with a Rayleigh distribution, and an L value of length L in the Viterbi equalizer is estimated, eight. Modulation is BPSK and burst length is and the difference between the received signal 200 bits. and the replica signal based on this model is compared and the sequence that minimizes the A Simplified List-Output Viterbi Equalizer error power is given as the decision sequence. Fig. 2 shows the configuration of a compare- In the optimum structure, the Viterbi equal- select operation for a list-output Viterbi equalizer[5] izer provides maximum likelihood sequence es- that has been simplified as follows: some degree timation (MLSE)[1]. The error value is referred of selection errors are permitted, and if the upper to a “metric.” A squared metric is normally level S path can be selected, the list need not be used, although a modified square has also been sorted into path metric order. The compare-select proposed.[2] operations of a simplified list-output Viterbi equalizer with S = 8 has approximately the same delay List-Output Viterbi Algorithm as MLSE. The complexity of the MLSE grows exponen- Fig. 3 shows the bit-error rate (BER) perfor- tially with the channel memory length. The mance of a simplified list-output Viterbi equal- problem is solved by the list-output Viterbi izer. The “sorting” type shown is a conventional equalizer.[3] The generalization of this device is list-output Viterbi equalizer, the “fast” type is as follows. The trellis memory length V is a simplified list-output Viterbi equalizer. The

*Hiroshi Kubo and Keishi Murakami are with the Information Technology R&D Center.

12 · Mitsubishi Electric ADVANCE TECHNICAL REPORTS

Metric 2S S 1 Key : Squared metric, V = 3 : Modified metric, V = 3 Input 2S to S Selected ... selector ... : Metric selection, V = 3 value value 10-1 : Optimum metric, V = 3 ... : MLSE, V = 8

10-2 Metric conversion ... Selection ... data BER 10-3 S Simplified 2S to S comparator 10-4

Fig. 2 Compare-select processor for a simplified list-output Viterbi equalizer. 10-5 5 10 15 20 25 Average Eb/N0 (dB) Fig. 4 BER performance of a Viterbi equalizer 1 with metric selection. Key V S Fast Sorting 0 8 -1 10 1 4 tion method yields better BER characteristics 2 2 than either of the individual metrics over re- 3 1 gions with a good S/N ratio, and performance 10-2 8 1 close to that of the ideal metric is obtained.

BER With society’s increased dependence on net- 10-3 work connectivity, it is natural that these abili- ties be extended to mobile users of information

10-4 equipment. Broadband mobile communication systems technologies, such as the Viterbi equalizer described here, help to make a wider data 10-5 channel available for communications by pro- 5 10 15 20 25 cessing the signal to suit the varying character- Average Eb/N0 (dB) istics of the wireless transmission link between Fig. 3 BER performance of a simplified list- mobile user and base station. ❑ output Viterbi equalizer. References 1. Forney, Jr., G.D., “Maximum-likelihood sequence estimation of digital sequences in the presence of intersymbol interference,” number of survivor paths for the overall algo- IDD Trans. Inform. Theory, IT-18, No. 3, 363 - 378, 1972. 2. Ungerbocck, G., “Adaptive maximum-likelihood receiver for rithm has been set at eight. The figure indi- carrier-modulated data-transmission systems,” IEEE Trans. cates that the increase in the number of survivor Commun., COM-22, No. 5, 624-636, 1974. paths S in each state improves the BER perfor- 3. Hashimoto, T., “A list-type reduced-constraint generation of the Viterbi algorithm, IEEE Trans. Inform. Theory IT-33, No. 6, 866 - mance, so that performance degradation of the 876, 1987. simplified processing is minimal. 4. Duel-Hallen, A. and Heegard, C., “Delayed decision-feedback sequence estimation, IEEE Trans. Commun., COM-37, No. 5, 428 - 436, 1989. A Metric SelectionViterbi Equalizer 5. K. Tanada, H. Kudo, A. Iwase, K. Murakami and T. Fujino: "An Squared metric and modified metric criteria adaptive list-output Viterbi equalizer with simplfied compare-select operation," ISITA'98, Oct. 1998 (submitted). have different characteristics for list-output 6. H. Kubo, K. Murakami and T. Fujino: "A list-output Viterbi equalizer Viterbi equalizers. It is possible to improve the with two kinds of metric criteria," in Proc. of IEEE ICUPC'98, Oct. performance of a Viterbi equalizer by estimat- 1998 (accepted). ing the channel quality by a training sequence in each slot and then choosing the metric crite- rion with the better match or some weighted combination of the two.[6] Fig. 4 shows the BER of a Viterbi equalizer using metric criteria selection, with trellis memory length V = 3. The proposed combina-

September 1998 · 13 TECHNICAL REPORTS An Improved Differential Detection Scheme Based on Maximum Likelihood Sequence Estimation

by Toshiharu Kojima and Makoto Miyake*

Although differential detection is a suitable de-

modulation scheme for mobile communications, ∆θ θ ai i i si Transmitted Mapping Differential Phase its bit error rate (BER) performance in an additive symbol encoding mod white Gaussian noise (AWGN) channel is infe- n ^ i a^ ∆θ r rior to that of a coherent detection scheme. To Demodulated i i Differential i Decision phase remedy this deficiency, Mitsubishi Electric has symbol estimation developed an improved differential detection scheme using maximum likelihood sequence estimation based on the Viterbi algorithm. The Fig. 1 Communication system model. resultant scheme achieves good BER performance not only in AWGN channels but also in fast Rician fading channels and slow Rayleigh fading channels.

Multiple ^ Θ λ Sequence ∆θ r Branch i Estimated Received i symbol i i estimation The improved differential scheme developed[1] by signal differential metric differential calculation (Viterbi phase phase algorithm) Mitsubishi Electric is a multiple differential de- detection tection scheme that improves the BER performance by estimating demodulation data from 1, [2,3] ..., N(22) differential detection signals. The Fig. 2 Configuration of multiple differential method involves differential detection of phase phase detection scheme. changes in the specified symbol interval of the received signal. We call the developed scheme a

“multiple differential phase detection” (MDPD) ceived signal sequence {ri} and produces the scheme, since its algorithm only requires phase demodulated symbol sequence {âi}. information of the received signal. Configuration of the Multiple Differential Communication System Model Phase Detection Scheme Fig. 1 shows the communication system model. Fig. 2 shows the MDPD scheme configuration.

In the modulator, an M-valued transmitted sym- In this scheme, 1, ..., N(22) symbol differential θ θ bol ai is Gray mapped to a transmitted phase signals (1)i,... (N)i are generated from the re- ∆θ ∈ differential phase i S, where the set of trans- ceived signal ri by multiple symbol differential π mitted differential phases S = {2m /M; with m phase detection. When ri is represented by θ θ θ θ = 0, 1,... M-1}. Ri exp(-j (0)i), (n)i is represented by (0)i - (0)i-n. θ Then, the transmitted signal phase i is gen- Because of the differential encoding of Eq. 1, θ erated by differential encoding: the N-symbol differential detection signal (N)i contains information on a partial sequence of θ θ ∆θ i = i-1 + i ...... (Eq. 1) length N of the transmitted differential phase ∆θ sequence { i}. By using this property, the √ θ and the transmitted signal si = Es exp(-j i) of MDPD scheme estimates the transmitted dif- ∆θ the differential MPSK signal is transmitted, ferential phase sequence { i} from the differ- Θ θ θ where Es = Eb log2M, and Eb is the signal energy ential phase symbol i = ( (1)i,... (N)i) composed per bit. of the 1, ..., N-symbol differential detection sig- In the channel path, AWGN ni with the single- nals. side noise power density No is added to the trans- The sequence estimation is performed by mitted signal si, resulting in the received signal Viterbi algorithm with the trellis diagram hav- N-1 ri. The demodulator estimates the transmitted ing M states composed of N-1 elements of ∆θ differential phase sequence { i} from the re- the set S of the transmitted differential phases.

*Toshiharu Kojima and Makoto Miyake are with the Information Technology R&D Center.

14 · Mitsubishi Electric ADVANCE TECHNICAL REPORTS

10-1 10-1

10-2

10-3 BER BER

10-4

Simulation results 10-3 Ideal coherent detection Simulation results Multiple differential phase detection (N=4) Ideal coherent detection 10-5 Multiple differential phase detection (N=3) Multiple differential Multiple differential phase detection (N=2) phase detection (N=3) Differential detection Differential detection

-4 10-6 10 2 4 6 8 10 12 2 4 6 8 1012 14 16

Eb/N0 (dB) Eb/N0 (dB)

Fig. 3 Simulated BER performance in AWGN Fig. 4 Simulated BER performance in fast Rician channel using differential QPSK fading channel using differential QPSK

modulation. Solid lines indicate modulation with C/M = 10dB and fDTs = 5 theoretical values. × 10-2. Solid lines indicate theoretical values.

Consider the state transition on the trellis BER Performance Φ φ diagram from the state where i-1 = ( i-N+1, We will now describe the BER characteristics φ φ Φ φ i-N+2,... i-1) at time (i-1) to a state i = ( i-N-2,... of a differential QPSK modulated signal pro- φ φ φ ∈ i-1, i), at time i where i S. The branch met- cessed using multiple differential phase detec- λ Φ Φ ric ( i-1, i) corresponding to this transition is tion, as measured by computer simulation. given by Nn-1 BER PERFORMANCE IN AWGN CHANNEL. Fig. 3 λ Φ Φ ∑|θ − ∑φ | ...... ( i-1, i)= (n)i i-k (Eq. 2) shows the BER performance in AWGN chan- n=1 k=0 nel. The MDPD scheme clearly improves the θ θ The differential detection signal (1)i,... (N)i for BER performance of the differential detection. 1 to N symbols includes the independent noise The improvement in gain of BER performance ni-1,... ni-N, so that the branch metric generation increases with the total number of differential process has the effect of averaging the noise. As detection signals N. When N = 4, BER perfor- a result, the branch metric has a higher signal- mance of the MDPD scheme close to that of to-noise power ratio than the original differen- coherent detection, i.e., within 0.3dB. tial detection signal. Therefore, the BER performance of the MDPD scheme represents BER PERFORMANCE IN FADING CHANNEL. Fig. 4 an improvement. shows BER performance in fast Rician fading

September 1998 · 15 TECHNICAL REPORTS

communications not only in additive white Gaussian noise channels but also in channels ❑ 10-1 affected by both fast and slow fading.

References 1. T. Kojima, M. Miyake and T. Fujino, “Differential detection scheme for DPSK using phase sequence estimation,” Trans. IEICE B-II, vol. 10-2 J76-B-II, no.10, pp. 783 - 792, Oct. 1993. (in Japanese) 2. S. Samejima, K. Enemoto and Y. Watanabe, “Differential PSK system with nonredundant error correction,” IEEE J. Select. Areas Commun., vol. SAC-1, no. 1, pp. 74 - 81, Jan. 1983. 3. D. Divsalar and M.K. Simon, “Multiple-symbol differential detection 10-3 of MPSK,” IEEE Trans. Commun., vol. 38, no. 3, pp. 300 - 308, March 1990. BER

10-4

Simulation results Multiple differential phase 10-5 detection (N=3) Differential detection

10-6 0102030405060

Eb/N0 (dB)

Fig. 5 Simulated BER performance in slow Rayleigh fading channel, using differential -4 QPSK modulation with fDTs = 1 x 10 . Solid lines indicate theoretical values.

with C/M (the Rice factor) = 10dB. In this case, the maximum Doppler frequency normalized -2 by symbol duration fDTs is set to 5 × 10 . It is evident from Fig. 4 that the MDPD scheme improves the BER performance of differential detection by at least 1dB. It is also evident that BER performance in this case is close to that of ideal coherent detection, despite the difficulty of a fast fading channel. Fig. 5 shows BER performance in slow -4 Rayleigh fading with fDTs = 1 × 10 . The BER performance of the MDPD scheme is better than that of differential detection by 0.7dB.

The improvement in BER performance achieved by Mitsubishi Electric’s successful development of a multiple differential phase detection system promises to improve the quality of digital

16 · Mitsubishi Electric ADVANCE TECHNICAL REPORTS Low Bit-Rate Speed Coding Technology

by Hirohisa Tasaki and Shin’ya Takahashi*

Speech coding technologies for compressing Input digital speech signals are being explored to help speech LPC better utilize limited bandwidth resources and analysis to support communications privacy. This article g Excitation Synthesis Minimize reports on recent developments at Mitsubishi codebook filter distortion Coded speech

Electric on low bit-rate speech coding technol- Gain Control Multiplex ogy which is central to cellular telephones and has numerous other applications. a) Encoder Speech coding technology provides the voice bandwidth compression used in cellular tele- g phone systems worldwide. The compressed sig- Coded Excitation Synthesis Output speech codebook filter speech nal has a bit rate of 3.45~13kbps, which increases to 5.6~22.8kbps when error correction De-multiplex is added. Most of these methods employ some b) Decoder type of the code excited linear prediction (CELP) algorithm illustrated in Fig. 1. This algorithm Fig. 1 Block diagram of CELP speech coder. involves dividing the speech signal into 20~40ms frames, and then representing each frame by the best approximation of sound vec- Table 1 Bit Allocation (bits/20ms) tors from an excitation codebook. Only the in- Mode Parameter dices of these vectors are actually transmitted. Basic Transient Although codebooks can be designed to capture LSP 18 18 the characteristics of speech, extensive calcu- lations are needed to search for the best mix of Power 6 6 vectors to express speech at low bit rates. Stud- Mode flag 1 x 2 1 x 2 ies on codebooks and search methodologies are ACB/FCB 8 x 2 — being conducted to reduce the computational SCB (6 + 6) x 2 — requirements and increase the sound quality, while other studies are being conducted on in- Algebra position — (4 + 4 + 4 + 5) x 2 creasing system robustness toward channel er- Algebra sign (1 + 1) x 2 (1 + 1 + 1 + 1) x 2 rors and background noise. Gain 5 x 2 6 x 2 Total 80 80 Pitch-Position Synchronized CELP Mitsubishi Electric has investigated pitch- position synchronized (PPS) CELP, in which the efficiency of the excitation signal representa- codebook with fixed components and an adap- tion is improved by use of an adaptive scheme tive codebook containing prior excitation com- based on the pitch peak position estimated by ponents. Pitch-position selection involves using prior samples. Fig. 2 shows the configu- choosing stochastic excitation vectors based on ration of the PPS-CELP excitation signal gen- adaptive speech coding values. The head of the eration system and Table 1 the bit allocation. pulse train is positioned to minimize the dis- The coding unit divides each frame into two tance between the synthetic signals created by subframes, encodes basic and transient modes adaptive excitation and the pulse train with the for each, and selects the mode with lower dis- same interval as adaptive excitation. Called the tortion. pitch position point, this position is used as the In the basic mode, excitation vectors are se- center for stochastic excitation vector. A recent lected as a sum of vectors selected from a static improvement has been to replace part of the

*Hirohisa Tasaki and Shin’ya Takahashi are with the Information Technology R&D Center.

September 1998 · 17 TECHNICAL REPORTS

CODEBOOK 1

Adaptive codebook

Fixed codebook

Extract pitch position

CODEBOOK 2

Stochastic Basic codebook 1 mode Synchronization Stochastic codebook 2

Stochastic codebook 3 Excitation signal Stochastic codebook 4 Transient mode

Algebraic codebook

Fig. 2 Excitation signal generation of PPS CELP.

stochastic excitation codebook with pulse ex- Table 2 Objective Test Results citation. Partial pulse Transient Segmental CD Conditions In the transient mode, algebraic excitation is codebook mode SNR (dB) employed, with only pulse position and polar- 1 No No 7.61 2.43

ity used to encode the excitation signals. The 2 Yes No 8.09 2.41 basic mode has good coding characteristics under ordinary conditions where the pitch pe- 3 No Yes 8.13 2.31 riodicity is high. Severe distortion occurs when 4 Yes Yes 8.38 2.30 major changes in sound level occur, leading to introduction of the transient mode, in which the optimum amplitude for each single pulse is taken as the provisional gain for conducting a Table 3 Subjective Test Results pulse-position search. Conditions Female voice Male voice Average Table 2 shows the results of tests on the ef- 4 − 1 +0.27 +0.53 +0.40 fectiveness of using stochastic excitation with partial pulse excitation in conjunction with a transient mode. The segmental signal-to-noise ratio (SNR) is improved and the cepstrum dis- tance is reduced. measured in terms of the average opinion score, Table 3 shows the results of subjective sound was 0.4. There are significant improvements quality evaluation of signals coded under con- for a male voice. Subjectively, the listeners re- ditions 1 and 4 from Table 2. Ten sentences were ported that the noise was lower and consonant heard by 16 listeners, who evaluated the sound sounds clearer with the improved coding algo- quality on a five-point scale. The improvement, rithm.

18 · Mitsubishi Electric ADVANCE TECHNICAL REPORTS

lacks flexibility, and is not easily adapted to

Key various design requirements. We have therefore Input LSP investigated calculating line spectrum pairs Case 1 from the linear prediction coefficients, and then Case 2 Case 3 compensating the pairs to determine the filter coefficients.[1] Fig. 3 shows typical characteristics. Compared with the conventional post filtering of Case 1, Cases 2 and 3 emphasize dynamic range at

Logged power spectra Logged power higher frequencies without affecting the low- end frequency response. This approach yields audible improvements in the speech clarity and also permits fine-grained control over the sound

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 quality of demodulated audio signals. Frequency (Hz) The speech coding technology method pre- Fig. 3 Examples of spectrum after postfilters. sented here combines a number of techniques to achieve substantial incremental improvement in both subjective and objective measures Error Control under PPS CELP of sound quality. These improvements also re- Since the position synchronization of the sto- duce processor and bandwidth requirements, chastic excitation and the position control of promising a substantial downward impact on ❑ codebook vectors depend on previous coding telecommunications costs. results, the effect of channel errors is a signifi- Reference cant consideration. Channel errors have a par- 1. Tasaki, H., Shiraki, K. and Takahashi, S. “Spectral Postfilter Design Based on LSP Transformation,” Proceedings of IEEE Speech ticularly large impact on stochastic coding when Coding Workshop, 1997. data redundancy using error correction codes is not employed. We investigated two error-resis- tant modifications. First, we extracted the pitch position synchronization points from the fixed excitation signals. Second, vector selection was centered on the fixed position, and then circu- lar shifted using synchronization point information. These measures brought about an approximately 0.4dB improvement in the segmental SNR under random bit errors, with virtually no signal degradation under error-free conditions.

Post Processing We introduced a spectral post-processing function that reduces the CELP algorithm charac- teristic distortion in the demodulated audio signal by emphasizing the peaks and valleys of the output spectrum. Filter coefficients are generally determined by simply squaring the linear prediction coefficients; however, this approach

September 1998 · 19 SPECIAL REPORT Recent Topics in the ITU-R Relating to Mobile Radio Communications

by Masayoshi Murotani*

Headquartered in Geneva, Switzerland, the In- a year, and progress is gradual at best. A meeting ternational Telecommunication Union (ITU) is in South Korea in 1997 established a procedure to responsible for international regulation and nominate standards for the radio transmission standardization in the telecommunications technology portion of the standard, and proposals field. In 1985, the ITU’s Radiocommunication are now being prepared. Possible candidates are Sector (ITU-R) initiated development of the narrow-band TDMA, wide-band TDMA, narrow- mobile telephone standards underlying the ex- band CDMA and wide-band CDMA. Wide-band plosive worldwide growth of cellular phone and CDMA appears the most promising. Japan has other mobile radio services. ITU-R’s Task Group submitted a proposal based on wide-band 8/1, responsible for mobile radio services, is CDMA. Unanimously agreeing to a single stan- working towards standards for a globally uni- dard will be a great achievement. While all na- fied terrestrial mobile system with completion tions agree in principle to the value of a targeted for the year 2000. ITU-R is also devel- worldwide standard, deep disagreements remain oping a worldwide mobile communications to be resolved. systems using a network of low earth orbiting The standardization process is arduous. There satellites (LEO). The ITU-R is concerned with are more than 700 ITU-R recommendations both regulations and standards. Reference 1 dealing with standardization of various kinds describes various activities of the ITU-R related of radiocommunication systems. None of them to mobile radiocommunications until 1994. are complete. This article presents various developments in Issues of intellectual property rights also this field since 1994.[1] present a barrier to worldwide standardization. Free use of all patents related to a candidate IMT-2000, a Unified Terrestrial Mobile standard would contribute greatly to building Telephone System consensus on a unified standard, but such shar- In 1985, TG 8/1 began working on a unified ing is not possible in the current competitive terrestrial mobile telephone system that would business environment and the ITU’s mandate be designed to work with satellite based systems. does not extend to this area. The project is now called IMT-2000, referring On the other hand, we have to ask ourselves to standards for a 2GHz International Mobile who really wants to have a worldwide standard. Telephone system to be completed by the year Currently there are three mobile radio systems 2000. The use of a single operating frequency band in the world: the European system, the North worldwide—a central concept of the system—was American system and the Japanese system. agreed to in a weak resolution at the 1992 World Among them, one system has been adopted in Administrative Radio Conference (WARC-92) more countries than the other two. The devel- after long and difficult negotiations. Article opers of this most popular system would prefer S5.388 of the ITU’s Radio Regulations allocates to have their regional system adopted as the de the 1,885~2,025MHz and 2,110~2,200MHz facto worldwide standard. The regional stan- bands for the system but does not preclude the dardization bodies are highly influential, which use of these bands for other services. In fact, complicates the ITU’s task of consensus- several other applications have been proposed building. An early solution is unlikely in the for these frequencies. An unrelated satellite- opinion of the author, who has been involved based mobile telephone system has been planned in ITU activities for many years. for the 1,980~2,010MHz and 2,170~2,200MHz Another important aspect related to the IMT- bands, and the United States, Europe and Japan 2000 is an anticipated shortage of frequency are seeking to allocate the terrestrial-use portion spectrum. The available frequencies cannot sat- of the band to an unrelated terrestrial mobile isfy the growing demand for multimedia-capable system. wideband mobile communication systems. The TG 8/1 has been conducting meetings twice next World Radiocommunication Conference

*Masayoshi Murotani is an adviser with Corporate Research and Development.

20 · Mitsubishi Electric ADVANCE SPECIAL REPORT

(WRC) in 1999 will consider spectrum and regu- tailed discussion is needed to reach a consen- latory issues for advanced mobile applications sus. in the context of IMT-2000, noting that there is WRC-97 decided that studies on this issue an urgent need to provide more spectrum for should continue, and the proposal to share the terrestrial component of such applications broadcasting bands--previously off limits--re- and that priority should be given to terrestrial flects the strength of the demands for the satel- mobile spectrum needs (see WRC-97 Resolu- lite-based mobile telephone services. tion 721). Satellite Based Mobile Telephone Services in Mobile Communications by Satellite the 1-3 GHz range and their Feeder Links Recently various companies in the United States One of the major decisions at the 1995 World and Europe are promoting plans for mobile Radiocommunication Conference (WRC-95) radiocommunication networks with a global was the allocation of frequency bands for the coverage using low earth orbiting satellites feeder links of satellite-based mobile telephone (LEO). Two types of networks are proposed. One services. The downlinks of these services will uses the frequency bands below 1GHz and is use 6,700~7,075MHz, 15.4~15.7GHz and called “Little LEO” because it is mainly used 19.3~19.6GHz. The uplinks will occupy for low-speed message communication (such as the 5,091~5,250MHz, 15.45~15.65GHz, paging) in a low frequency band. The other uses 19.3~19.6GHz and 29.1~29.4GHz bands. the 1~3GHz band and is called "Big LEO" be- The 15.45~15.65GHz and 19.3~19.6GHz bands cause it is used for high-speed communication are for sharing as uplinks and downlinks. The including voice in a higher frequency band. frequency sharing situations have become very Several projects are underway for Little LEO complicated in other allocated bands. Frequency networks. One serious problem is that very few sharing criteria have been established for each frequency bands are available for the mobile- case. One example is the 6,700~7,075MHz satellite service in the bands below 1GHz. In band which is shared by microwave radio re- order to expand available frequency bands, stud- lay systems for fixed terrestrial services. Evalu- ies have been actively carried out on technologies ating interference of a large number of for using the bands without giving unacceptable nongeostationary satellites on terrestrial radio- interference to other services sharing the bands. relay systems is difficult, although Japan has One example is Dynamic Channel Activity As- gone as far as calculating the flux-density at signment System (DCAAS). In this system, the the Earth’s surface due to satellite transmis- spectrum usage at the surface of the Earth is sions considering the space-diversity reception continuously monitored by satellites and only widely used in radio-relay systems. those frequencies which are not used will be Nations at WRC-95 agreed to allocate used by satellites. At the WRC-97, one nation 1,980~2,010MHz and 2,170~2,200MHz bands in proposed the application of DCAAS to the the year 2000 to provide service links between 148~149.9MHz band which is now used for satellites and mobile user terminals. Protests mobile land services. This proposal was rejected. from developing countries, which will have to Some proposals recommend sharing tele- change the operating frequencies of their radio- vision broadcasting bands (470~582MHz relay systems, was overcome through offers of and 582~960MHz) and radio broadcasting technical cooperation from the advanced na- (45~70MHz, 88~108MHz and 170~240MHz) tions. with mobile satellite communication ser- Frequency sharing criteria between the mobile- vices but the broadcasting industry unani- satellite service and terrestrial radio services mously opposes this. The author regrets that are generally determined in relation to fixed the opposing interests have not discussed the services such as radio-relay systems. However, issues face to face in the same room, since de- the 1,492~1,525MHz band allocated to the

September 1997 · 21 SPECIAL REPORT

mobile-satellite service in Region 2 (North and 500MHz in the year 2010. In contrast, the cur- South America) is one exception. WRC-95 de- rent allocation for the year 2000 allows only termined that the coordination threshold in 200MHz in Regions 1 (Europe and Africa) and 3 terms of the power flux-density levels at the (Asia and Pacific) and 280MHz in Region 2 surface of the Earth for space stations in the (North and South America). mobile-satellite (space-to-Earth) service, with Spectrum demand is also related with how respect to the land mobile service use within many different systems of mobile communica- the territory of Japan, shall be -50dB (W/m2) in tion networks using non-GSO satellites can any 4kHz band for all angles of arrival (see Ar- share the same frequency bands. It is generally ticle S5.384A of the Radio Regulations). This recognized that systems using FDMA or TDMA provision takes into account the wide use of cannot coexist with systems using CDMA this band in Japan for the terrestrial mobile when the service areas are the same and the communications including portable telephones. frequency bands are also the same. If both sys- This is an important provision from the Japa- tems use CDMA, frequency sharing seems fea- nese viewpoint. On the other hand, from an sible subject to certain conditions. This kind of international perspective, it reflects the current constraint seems to be an intrinsic problem of complicated spectrum usage. non-GSO mobile-satellite systems. Additional frequency allocations at WRC-97 Preparation for WRC-97 and its Decisions for the service links of the mobile-satellite ser- Further debate concerning additional allocation vice were modest, and the frequency shortage of frequency bands in the 1~3GHz range took is expected to continue. place in the preparatory phase for WRC-97, Important progress was made prior WRC-97 which was held in October and November 1997. with respect to frequency sharing between the Discussions continued from the ITU World satellite-based mobile services and the fixed Telecommunication Forum on Global Mobile service radio-relay systems in the 1~3GHz band. Personal Communications by Satellite (GMPCS) In 1995, Article ITU-R IS.1141 and IS.1142 es- which was held in 1996 and resolved that com- tablished emitted maximum power levels for petitive policy should be introduced also in mobile-service satellites for the purpose of co- GMPCS. Sufficient frequency allocation is es- ordinating with fixed services. In general, the sential to support competition; however, actual recommendations expressed the satellite power allocation is opposed by users of current alloca- over a 4kHz reference band; however, Article tions, since the new allocations inevitably af- ITU-R F.1246, approved in 1997, recommends a fect the quality of services in the existing 1MHz reference band for digital fixed service allocations. systems and combined 1MHz and 4kHz bands Congestion in the spectrum usage is first re- for analog fixed service systems. The article also flected in the advance notices which are sub- states that the level in the 1MHz reference band mitted to the ITU in order to negotiate with the should be 24dB higher than the current pre- affected nations several years before satellites scribed level for the 4kHz band and the level in are actually launched. As of May 1997, as many the 4kHz band should be 6dB higher than the as 243 notifications had been published for sat- current level. Article ITU-R F.1246 will facili- ellite based mobile communication systems for tate coordination to a certain degree. Detailed the 1~3GHz band. It is impossible for all of these interference assessments carried out in Japan projects to be actually implemented. Many may made a great contribution to the establishment be so-called “paper satellites” which are not in- of this recommendation. tended for actual launch. Nevertheless, the situ- One interesting aspect which has emerged in ation is difficult. One forecast estimates that the study concerning feeder links is the inter- the spectrum demand for the mobile-satellite pretation of Article S22.2 of the Radio Regula- services in the 1~3GHz range will be as large as tions which states that a non-GSO satellite shall

22 · Mitsubishi Electric ADVANCE SPECIAL REPORT

not cause harmful interference to GSO satel- has been reduced to about 200. Earth stations lites in fixed-satellite services. In view of the for the system must be engineered to automati- increasing number of non-GSO satellites in cally track the moving satellites. recent years, it has become a heated issue how The WRC-97 received a similar proposal from to apply Article S22.2 of the Radio Regulations. France which plans to implement non-GSO One interpretation is that GSO satellites have fixed-service system in other frequency bands an absolute priority over non-GSO satellites, including the 12GHz band which is now being including future GSO satellites, on which no used for the satellite-broadcasting services. coordination requirements would be imposed. Frequency sharing between non-GSO fixed- Another interpretation is that acceptable inter- service systems, geostationary fixed-service ference levels should be discussed by all parties systems, geostationary direct satellite broad- concerned and non-GSO satellites should cease casting services and and terrestrial services is a to operate only when the interference to exist- serious problem. Provisional requirements for ing GSO satellites exceeds the allowable level. controlling the mutual interference have been Although this issue continues to be debated, laid down in the Radio Regulations, but much the second interpretation has been prevailing work remains to be done. The ITU-R has cre- since WRC-97. ated a special Task Group to study this matter and the work is under way. New Technologies for Fixed Service Applications Fixed Services Using Platforms in the Fixed service means radiocommunications be- Stratosphere tween fixed points and mobile service means The January 1997 meeting of ITU-R Study radiocommunications involving mobile sta- Group 9 (fixed service) received a proposal from tions. While the definitions are different, the the United States to launch balloons at an alti- relevant technologies and applications are tude of 23km in the stratosphere for use as re- gradually merging. We therefore choose to in- peaters in the 47.2~50.2GHz (millimeter-wave) clude recent topics in fixed applications. band. Each of these balloon-lofted repeaters could support fixed service regions of up to sev- Fixed Services Using Non-Geostationary eral hundred km in radius. Mobile communi- Satellites cations would not be possible at such high Most prior fixed-service radiocommunication frequencies, and the U.S. chose this frequency systems have employed geostationary satellites. since lower-frequency bands—which would At WRC-95, the United States submitted a pro- have been preferred—were unavailable. The sys- posal for a fixed-service system using non-geo- tem is feasible due to development of high effi- stationary satellites. The proposed system ciency ion engines that can be used to control would employ as many as 840 of these satel- the platform position. It was reported that solar lites operating in the 20/30GHz bands to estab- cells can provide sufficient power for commu- lish a worldwide commercial information nication and position control. network. The U.S. presentation caused some- Representatives at the meeting initially agreed thing of an uproar since the topic was not a part to classify the new system as terrestrial, then of the agenda established two years before, but later officially categorized the system as high the other nations agreed to consider the pro- altitude platform stations (HAPS), incorporat- posal and ended up allocating the 18.8~19.3GHz ing the definition into the Radio Regulations. band for downlinks and 28.6~29.1GHz band for The 47.2~47.5GHz and 47.9~48.2GHz fre- uplinks. This project is now being managed by quency bands were designated for use by HAPS U.S. companies. providing fixed services. Eight hundred forty satellites seems exces- The proposed system still has many techni- sive, and unofficial reports suggest the number cal difficulties to overcome and the feasibility

September 1998 · 23 SPECIAL REPORT

of a several hundred km radius service area at 47GHz remains to be demonstrated. Even if technical difficulties are overcome, the commercial viability of the venture is uncertain, and the project is going forward at some risk to its investors.

Conclusion The ITU-R is now at a most crucial stage in establishing IMT-2000 as a global standard for mobile communications. Several satellite-based mobile communication systems are currently being implemented and various technical standards have been established, but significant issues, including the shortage of spectrum are still unresolved. Meanwhile, fixed-service radiocommunication projects are under way to serve similar markets. Technologies are combining and the market is becoming more competitive. ❏

Reference 1. M. Murotani, "International activities affecting the mobile radiocommunication frequency spectrum," Advance, Vol.73, December 1995.

24 · Mitsubishi Electric ADVANCE MITSUBISHI ELECTRIC OVERSEAS NETWORK (Abridged)

Country Address Telephone U.S.A. Mitsubishi Electric America, Inc. 5665 Plaza Drive, P.O. Box 6007, Cypress, California 90630-0007 714-220-2500 Mitsubishi Electric America, Inc. Sunnyvale Office 1050 East Arques Avenue, Sunnyvale, California 94086 408-731-3973 Mitsubishi Electronics America, Inc. 5665 Plaza Drive, P.O. Box 6007, Cypress, California 90630-0007 714-220-2500 Mitsubishi Consumer Electronics America, Inc. 9351, Jeronimo Road, Irvine, California 92618 949-465-6000 Mitsubishi Semiconductor America, Inc. Three Diamond Lane, Durham, North Carolina 27704 919-479-3333 Mitsubishi Electric Power Products Inc. 512 Keystone Drive, Warrendale, Pennsylvania 15086 724-772-2555 Mitsubishi Electric Automotive America, Inc. 4773 Bethany Road, Mason, Ohio 45040 513-398-2220 Astronet Corporation 3805 Crestwood Parkway Suite 400 Duluth, Georgia 30096 770-638-2000 Powerex, Inc. 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Ltd. 348 Victoria Road, Rydalmere, N.S.W. 2116 2-9684-7777 New Zealand Melco New Zealand Ltd. 1 Parliament St., Lower Hutt, Wellington 4-560-9100 Representatives Korea Mitsubishi Electric Corp. Seoul Office Daehan Kyoyuk Insurance Bldg., Room No. 2205, #1,1-ka, Chongno-ku, Seoul 2-732-1531~2 India Mitsubishi Electric Corp. New Delhi Liaison Office Dr. Gopal Das Bhawan (8th Floor), 28 Barakhamba Road, New Delhi 110001 11-335-2343 Viet Nam Mitsubishi Electric Corp. Ho Chi Minh City Office 18th Floor, Sun Wah Tower, 115 Nguyen Hue Street, District 1, 8-821-9038 Ho Chin Minh City MITSUBISHI ELECTRIC CORPORATION HEAD OFFICE: MITSUBISHI DENKI BLDG., MARUNOUCHI, TOKYO 100-8310, FAX 03-3218-3455